The prior art has evolved surface profilers for scanning and profiling the surfaces of an object to determine the object's three-dimensional shape. Some surface profilers have touch probes which contact a surface of the object. Other surface profilers emit light or radiation (e.g. laser light) and detect its reflection from a surface.
Some light-emitting surface profiling systems have a laser scanner that is attached to a moveable arm equipped with encoders to determine relative motion of the scanner. The laser scanner is swept over an object and a camera takes images of the object's surface from multiple points of view as determined from the encoders. Reconstruction software processes the scanned data to produce three-dimensional surface maps of the object. However, the size of the moveable arm, the scanner and the camera inhibits use of such surface profiling systems to scan the interior of small objects.
Other light-emitting surface profiling systems for scanning the interior of objects have a laser mounted at the end of a stalk. Light emitted by the laser is incident on a target surface and imaged onto a camera by a lens positioned behind the laser. As the stalk tends to oscillate or move, accurate scanning measurements are difficult to obtain using such a surface profiling system.
As seen in FIG. 1A, triangulation may be used by light-emitting surface profilers to measure locations on a surface of an object which is illuminated by a light source, such as a laser 12, and obliquely viewed by a camera 14. Camera 14 incorporates an imaging sensor 16 for detecting light and a lens 15 for focusing and imaging light onto imaging sensor 16. In single point triangulation, laser 12 emits a laser beam 18 which is incident at a point (X,Y) on surface 10 of object 11. Laser beam 18 is reflected by surface 10 as laser beam 18′, and laser beam 18′ is imaged by lens 15 to a point (x,y) on imaging sensor 16. A laser beam 18 that is incident on another surface 10′ of object 11′ at a different point (X′,Y′) is reflected by surface 10′ as laser beam 18″ which is imaged to a different point (x′,y′) on imaging sensor 16. The relationship between locations on the surface of an object and locations on imaging sensor 16 is determined by the physical arrangement of laser 12 and camera 14. In particular, the distance between laser 12 and camera 14 (i.e. baseline 20) is fixed, as well as the angle θ between laser beam 18 and baseline 20. As such, the location of points (X,Y), (X′,Y′) on a surface can be determined from the measured location of points (x,y), (x′,y′), respectively, on imaging sensor 16.
Rather than illuminating a single point on a surface as seen in FIG. 1A, laser beam 18 may be spread so that a laser line 22 (i.e. a series of points) is projected onto surface 10, as seen in FIG. 1B. Line 22 is reflected and imaged as a corresponding line 23 on imaging sensor 16. The spreading of laser beam 18 advantageously allows more data points to be collected at once, as line 22 encompasses multiple points (X,Y) on surface 10 which are imaged on imaging sensor 16 as multiple points (x,y) on line 23. By fixing the physical arrangement of laser 12 and camera 14, triangulation techniques may be applied to determine the locations of points on line 22 on surface 10 from the measured locations of points on line 23 on imaging sensor 16.
The accuracy of light-emitting surface profiling systems using triangulation techniques may be affected by several factors. These include baseline shifts or variations. FIG. 2A illustrates the effect of moving camera 14 away from laser 12 by a distance s to define a new baseline 20′. As a result of this baseline shift, a point (X,Y) on surface 10—which would otherwise have been imaged at point (x,y) on imaging sensor 16 at baseline 20—is imaged at point (x′,y′) on imaging sensor 16 at the new baseline 20′. However, point (x′,y′) on imaging sensor 16 corresponds to a different point (X′,Y′) in the path of laser beam 18 at baseline 20. Therefore, a surface profiling system which is calibrated for a particular baseline 20 will not provide accurate measurements if camera 14 and/or laser 12 are moved to define a new baseline 20′. The difference Δd1 between points (X,Y) and (X′,Y′) corresponds to the error caused by the baseline shift.
Another factor which may affect the accuracy of light-emitting surface profiling systems is a shift in the direction of the light beam emitted by the light source. FIG. 2B illustrates the effect of changing the angle between laser beam 18 and lens axis 24 by an angle shift φ. As a result of this angle shift, light which would otherwise have been incident at point (X,Y) on surface 10 and imaged at point (x,y) on imaging sensor 16 is instead incident at point (X′,Y′) on surface 10 and imaged at point (x′,y′) on imaging sensor 16. The difference Δd2 between points (X,Y) and (X′,Y′) corresponds to the error caused by the angle shift.
There is a need for a surface profiler for scanning interior and exterior surfaces of objects in a range of shapes and sizes, which addresses the scanning inaccuracies and other disadvantages of prior art surface profilers.